Only two weeks old, with eyes barely opened and genders still undetermined, the two tiniest monkeys in Helensburgh were born with a thankfully well-developed instinct to hold on.

The pair closed their tiny fingers tight around the coat of their father Mitu yesterday as he raced along the raised Symbio Wildlife Park enclosure branches at his usual breakneck speed.

Mitu, in typical Cotton Top Tamarin style, is proving an involved and attentive father.

But he wasn't letting two tiny hangers-on slow him down, said head keeper Kylie Elliott.

"Mitu will still climb, run, jump as if there's nothing on his back," she said. "It's incredible to watch."

The infants, born on September 4, bring Symbio's critically endangered Cotton Top Tamarin population to nine, and are a bright new chapter for the family after a tragic beginning.

Mitu and his partner Bella, and the pair's firstborn twin sons Toro and Rico, were stolen from the park on May 30, 2010. The parents and Toro were later recovered but Rico was never found.

"Mitu and Bella didn't breed again for over 12 months," Ms Elliot said. "It took them several months for them to get their confidence back with us but they've gone on to produce three more litters since the theft, which shows the confidence in their home and their confidence in us, which is great to see."

The species rear co-operatively, so brothers and sisters are taught to look after younger monkeys, and that has busied Toro, the only one in the family without a sibling or partner since Rico went missing.

The family spent much of yesterday huddled around its new additions, who make kitten-like squeaking noises when they are hungry or are passed among minders.

The family grew increasingly secretive in the lead-up to the twins' birth but keepers are coming closer to the day they will be able to weigh the babies and determine their genders.